|Publication number||US6886237 B2|
|Application number||US 09/517,782|
|Publication date||May 3, 2005|
|Filing date||Mar 2, 2000|
|Priority date||Nov 5, 1999|
|Also published as||CN1244181C, CN1308385A, DE60029538D1, DE60029538T2, EP1098392A2, EP1098392A3, EP1098392B1, EP1691447A1, US7515115, US20030140480, US20050115056|
|Publication number||09517782, 517782, US 6886237 B2, US 6886237B2, US-B2-6886237, US6886237 B2, US6886237B2|
|Inventors||Oliver Paul Leisten, Peter Wileman|
|Original Assignee||Sarantel Limited|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (27), Non-Patent Citations (1), Referenced by (33), Classifications (19), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates to a method of producing an antenna, and primarily to a method of tuning a quadrifilar antenna for circularly polarised radiation at frequencies above 200 MHz. The invention also includes an antenna produced according to the method.
The backfire quadrifilar antenna is well-known and has particular application in the transmission and reception of circularly polarised signals to or from orbiting satellites. British Patent Application No. 2292638A discloses a miniature quadrifilar antenna having four half-wavelength helical antenna elements in the form of narrow conductive strips plated on the surface of a cylindrical ceramic core. Connecting radial elements on a distal end face of the core connect the helical elements to a coaxial feeder passing axially through the core in a narrow passage. The helical elements are arranged in pairs, the elements of one pair having a greater electrical length than those of the other pair by virtue of their following a meandering course, all four elements being connected to the rim of a conductive balun sleeve which rim describes a circle lying in a plane perpendicular to the antenna axis. British Patent Application No. 2310543A discloses an alternative antenna in which the balun sleeve has a non-planar rim, the helical elements being simple helices which terminate in peaks and troughs respectively of the rim in order to yield elements of the required different lengths.
The fact that the pairs of elements have different electrical lengths results in a phase difference between the currents in the respective pairs at the operating frequency of the antenna, and it is this phase difference which makes the antenna sensitive to circularly polarised radiation with a cardioid radiation pattern such that the antenna is suited to receiving circularly polarised signals from sources which are directly above the antenna, i.e. on the antenna axis, or at locations a few degrees above a plane perpendicular to the axis and passing through the antenna, or from sources located anywhere in the solid angle between these extremes. The radiation pattern is also characterised by an axial null in the direction opposite to the direction of maximum gain.
The bandwidth of the above described quadrifilar resonance is relatively narrow and, particularly in the case of miniature quadrifilar antennas having a core of a high dielectric constant, presents a manufacturing difficulty in achieving sufficiently close dimensional tolerances to be able repeatedly to produce antennas having the required cardioid response and resonant frequency.
According to a first aspect of this invention, there is provided a method of producing a quadrifilar antenna for circularly polarised radiation at frequencies above 200 MHz, the antenna comprising a plurality of substantially helical conductive radiating tracks located on an electrically insulative substrate, wherein the method comprises monitoring at least one electric parameter of the antenna and removing conductive material from at least one of the tracks to bring the monitored parameter nearer to a predetermined value, thereby increase the inductance of the track and to improve the circularly polarised radiation pattern of the antenna. In this way, it is possible to trim antennas in large scale production without resort to individual testing in, for instance, an electromagnetically anechoic chamber and without excessive manual intervention.
The preferred method involves removing the conductive material from the tracks by laser etching an aperture in one or more of the tracks, leaving the opposing edges of the affected tracks intact on either side of each aperture. The method is particularly applicable to an antenna in which the substrate is a substantially cylindrical body of ceramic material having a relative dielectric constant greater than 10, the tracks including portions on a cylindrical surface of the substrate and, in addition, on a flat end surface of the substrate substantially perpendicular to the cylinder axis. In this case, the conductive material is removed from track portions located on the flat end surface which, in the preferred antenna, is close to the feed point for the antenna elements and is a location of a voltage minimum at the quadrifilar resonance. In alternative embodiments, the aperture or apertures may be cut in positions of other voltage minima, for example, where the helical elements join a common linking conductor such as a balun sleeve encircling the core.
The monitoring step typically comprising coupling the antenna to a radio frequency source which is arranged to sweep a band of frequencies containing the operating frequency, and monitoring the relative phases and amplitudes of signals picked up by probes brought into juxtaposition with the tracks at predetermined locations such as the end portions of the tracks remote from the feed point. Preferably, the probes are capacitively coupled to the respective tracks to avoid the need for individual ground connections to the antenna.
The apertures formed in the tracks are preferably rectangular, each having a predetermined width transverse to the direction of the track, the width being computed automatically in response to the result of the monitoring step. This is a non-linear adjustment process, in that the inductance of the track added by the aperture is non-linearly related to the aperture area, and specifically to the width of the rectangular aperture. Computation of the aperture size is performed so as to bring the phase difference of the currents and/or voltages in the tracks of respective track pairs nearer to 90° and to adjust the frequency at which this orthorgonality occurs so as to be nearer the intended operating frequency.
The invention also includes, according to a second aspect, a quadrifilar antenna for circularly polarised radiation at frequencies above 200 MHz, comprising a plurality of substantially helical conductive tracks located on an electrically insulative substrate, wherein at least one of the tracks has a cut-out of predetermined size for increasing the inductance of the track. The preferred antenna has a substrate comprising an antenna core formed of a solid dielectric material, the tracks being arranged to as to define an interior volume the major part of which is occupied by the solid material of the core, wherein the substrate has curved outer surface portions and flat surface portions supporting the conductive tracks, and with each cut-out being formed where the respective track lies over the one of the flat surface portions.
The invention will be described below by way of example with reference to the drawings.
In the drawings:
The quadrifilar antenna described below is similar to that described in the above-mentioned British Patent Application No. GB2310543A, the disclosure of which is incorporated in this specification by reference. The disclosure of the above-mentioned related Application No. GB2292638A is also incorporated in this specification by reference.
The four longitudinally extending elements 10A-10D are of different lengths, two of the elements 10B, 10D being longer than the other two 10A, 10C by virtue of extending nearer the proximal end of the core 12. The elements of each pair 10A, 10C; 10B, 10D are diametrically opposite each other on opposite sides of the core axis.
In order to maintain approximately uniform radiation resistance for the helical elements 10A-10D, each element follows a simple helical path. The upper linking edge 20U of the sleeve 20 is of varying height (i.e. varying distance from the proximal end face 12P) to provide points of connection for the long and short elements respectively. Thus, in this embodiment, the linking edge 20U follows a shallow zig-zag path around the core 12, having two peaks and two troughs where it meets the short elements 10A, 10C and long elements 10B, 10D respectively, the amplitude of the zig-zag being shown in
Each pair of helical and corresponding connecting radial element portions (for example 10A, 10AR) constitutes a conductor having a predetermined electrical length. Each of the element pairs 10A, 10AR; 10C, 10CR having the shorter length produces a shorter transmission approximately 135° at the operating wavelength than each of the element pairs 10B, 10BR; 10D, 10DR. The average transmission delay being 180°, equivalent to an electrical length of λ/2 at the operating wavelength. The differing lengths produce the required phase shift conditions for a quadrifilar helix antenna for circularly polarised signals specified in Kilgus, “Resonant Quadrifilar Helix Design”, The Microwave Journal, December 1970, pages 49-54. Two of the element pairs 10C, 10CR; 10D, 10DR (i.e. one long element pair and one short element pair) are connected at the inner ends of the radial elements 10CR, 10DR to the inner conductor 18 of the feeder structure at the distal end of the core 12, while the radial elements of the other two element pairs 10A, 10AR; 10B, 10BR are connected to the feeder screen formed by outer screen 16. At the distal end of the feeder structure, the signals present on the inner conductor 18 and the feeder screen 16 are approximately balanced so that the antenna elements are connected to an approximately balanced source or load, as will be explained below. It will be appreciated that, in the general case, the tracks formed by the track portions 10A-10D and 10AR-10DR may have an average electrical length of nλ/2 where n is an integer and each may execute n/2 turns about the antenna axis 24.
With the left handed sense of the helical paths of the longitudinally extending elements 10A-10D, the antenna has its highest gain for right hand circularly polarised signals.
If the antenna is to be used instead for left hand circularly polarised signals, the direction of the helices is reversed and the pattern of connection of the radial elements is rotated through about 90°. In the case of an antenna suitable for receiving both left hand and right hand circularly polarised signals, the longitudinally extending elements can be arranged to follow paths which are generally parallel to the axis.
The conductive sleeve 20 covers a proximal portion of the antenna core 12, thereby surrounding the feeder structure 16, 18, with the material of the core 12 filling the whole of the space between the sleeve 20 and the metallic lining 16 of the axial passage 14. The sleeve 20 forms a cylinder connected to the lining 16 by the plating of the proximal end face 12P of the core 12. The combination of the sleeve 20 and the plating 22 forms a balun so that signals in the transmission line formed by the feeder structure 16, 18 are converted between an unbalanced state at the proximal end of the antenna and an approximately balanced state at an axial position generally at the same distance from the proximal end as the upper linking edge 20U of the sleeve 20. To achieve this effect, the average sleeve length is such that, in the presence of an underlying core material of relatively high relative dielectric constant, the balun has an average electrical length in the region of λ/4 at the operating frequency of the antenna. Since the core material of the antenna has a foreshortening effect, and the annular space surrounding the inner conductor 18 is filled with an insulating dielectric material having a relatively small dielectric constant, the feeder structure distally of the sleeve 20 has a short electrical length. Consequently, signals at the distal end of the feeder structure 16, 18 are at least approximately balanced.
The trap formed by the sleeve 20 provides an annular path along the linking edge 20U for currents between the elements 10A-10D, effectively forming two loops of different electrical lengths, the first with short elements 10A, 10C and the second with the long elements 10B, 10D. At quadrifilar resonance current maxima and voltage minima exist at the ends of the elements 10A-10D and in the linking edge 20U. The edge 20U is effectively isolated from the ground connector at its proximal edge due to the approximate quarter wavelength trap produced by the sleeve 20.
The antenna has a main quadrifilar resonant frequency for circularly polarised radiation in the region of 1575 MHz, the resonant frequency being determined by the effective electrical lengths of the antenna elements and, to a lesser degree, by their width. The lengths of the elements, for a given frequency of resonance, are also dependent on the relative dielectric constant of the core material, the dimensions of the antenna being substantially reduced with respect to an air-cored similarly constructed antenna.
The preferred material for the core 12 is zirconium-titanate-based material. This material has a relative dielectric constant in excess of 35 and is noted also for its dimensional and electrical stability with varying temperature. Dielectric loss is negligible. The core may be produced by extrusion or pressing.
The antenna elements 10A-10D, 10AR-10DR are metallic conductor tracks bonded to the outer cylindrical and end surfaces of the core 12, each track being of a width at least four times its thickness over its operative length. The tracks may be formed by initially plating the surfaces of the core 12 with a metallic layer and then selectively etching away the layer to expose the core according to a pattern applied in a photographic layer similar to that used for etching printed circuit boards. In all cases, the formation of the tracks as an integral layer on the outside of a dimensionally stable core leads to an antenna having dimensionally stable antenna elements. The circumferential spacing between the helical track portions is greater than (preferably more than twice) their width.
To achieve a radiation pattern having, a good front-to-back ratio together with acceptable gain and to achieve this radiation pattern at the required operating frequency, the antenna as described above and shown in
A better understanding of the way in which the antenna operates and the affect of the apertures will be obtained by referring to the graph of FIG. 4.
The diagram of
Naturally, the centre frequency indicated by dotted line 36 should correspond to the required operating frequency of the antenna as well.
It is possible by adjusting the inductance of one or more of the tracks 10A, 10AR, etc. to align or trim the antenna to achieve the phase orthogonality and centre frequency referred to above. For instance, the divergence of the phases at the centre frequency can be reduced by increasing the inductance of the shorter tracks 10A, 10AR and 10C, 10CR. The centre frequency can be reduced by increasing the inductance of all four tracks. It follows that to make full use of the adjustment facility provided by cutting apertures, the antenna should, initially, be manufactured so as to have tracks which are electrically shorter than the optimum lengths at the required operating frequency.
These concepts may be used, in accordance with the invention, as the basis for an automated antenna trimming process to reduce or eliminate the deviation in the antenna electrical parameters (such as signal phase and amplitude in the radiating element) from the required optimum values. In this way, it is possible to manufacture antennas relatively cheaply using an initial low tolerance manufacturing process without resort to expensive and labour-intensive manufacturing and trimming methods.
A test arrangement for performing the phase and amplitude measurements will now be described with reference to
It will be noted that in
During the test process, all four probes 42A-42D are brought into contact with the antenna 40, a swept radio frequency signal is applied to the antenna from output 48 of the test unit 56, and the probe signals received at inputs 54A to 54D are monitored. A centre frequency is computed by detecting the intersections of the amplitude characteristics (as described above with reference to
In effect, the test unit computes a crossover frequency representing the closest convergence of the four amplitude traces, marks the corresponding frequency, reads the four phase values at that frequency to compute the phase differences, and then computes the required added conductance for each track in order to shift the crossover frequency to the required frequency (in this case the GPS frequency of 1575.5 MHz) with the correct phase orthogonality. This is performed by calculating an LC (inductance X capacitance) product for each track.
The required aperture size is then computed and the laser is controlled to etch the aperture or apertures.
The invention can include a method of producing a quadrifilar antenna for circularty polansed radiation at frequencies above 200 MHz, the antenna comprising a plurality of substantially helical conductive radiating tracks located on an electrically insulative substrate, wherein the method comprises monitoring at least one electrical parameter of the antenna and removing conductive material from at least one of the tracks to increase the inductance of the track and thereby to bring the monitored parameter nearer to a predetermined value, wherein material is removed from the tracks by forming a rectangular aperture in each affected track, the aperture having a predetermined width transverse to the direction of the track which is computed automatically in response to the result of the monitoring step. In an embodiment of this method, the width and length of the aperture are variable in response to the said monitoring result.
The antenna may then be automatically removed from the test location shown in
In instances of the antenna being small compared to the probes, it is preferred that the relative dielectric constant of the antenna core is at least 10, and is preferably 35 or higher, in order that the probes do not materially affect the antenna characteristics during the above-described test.
The capacitive probes pick up signals representative of the very near field and are, therefore, able to provide signals corresponding to the currents in the individual tracks. This allows deduction of the far field pattern, in accordance with the phase relationships described above.
The removal of material is preferably performed by a pulsed YAG laser which allows metal ablation substantially without melting so as to provide precise dimensional control.
It is possible to form the apertures in the tracks at alternative positions, such as in the proximal end portions of the track portions 10A to 10D, providing alternative probe locations are chosen.
It will be understood that while this invention has been described by reference to a method of producing a quadrifilar antenna, the method may also be applied to other wire antennas (i.e. antennas having conductors which are narrow compared to the spacing between them).
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|U.S. Classification||29/600, 29/847, 29/602.1, 216/65, 29/593, 29/852, 343/895|
|International Classification||H01Q11/08, H01Q21/26, H01Q1/38, H01Q9/27|
|Cooperative Classification||Y10T29/4902, Y10T29/49156, H01Q11/08, Y10T29/49016, Y10T29/49018, Y10T29/49165, Y10T29/49004|
|Apr 3, 2000||AS||Assignment|
Owner name: SYMMETRICOM, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:LEISTEN, OLIVER PAUL;WILEMAN, PETER;REEL/FRAME:010737/0601;SIGNING DATES FROM 20000229 TO 20000320
|Jul 9, 2001||AS||Assignment|
Owner name: SARANTEL LIMITED, UNITED KINGDOM
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:SYMMETRICOM, INC.;REEL/FRAME:011958/0630
Effective date: 20010531
|Oct 14, 2008||FPAY||Fee payment|
Year of fee payment: 4
|Feb 29, 2012||AS||Assignment|
Owner name: HARRIS CORPORATION, NEW YORK
Free format text: SECURITY AGREEMENT;ASSIGNOR:SARANTEL LIMITED;REEL/FRAME:027786/0471
Effective date: 20120229
|Oct 5, 2012||FPAY||Fee payment|
Year of fee payment: 8